Closed respiratory systems are used when it is not possible for an individual to breath air normally. An example of a closed respiratory system is the air tank, air hose, regulator and mouthpiece used by a scuba diver to breath underwater. A problem for users of the closed system is that air that is inhaled from an air tank is generally devoid of moisture. Moisture is needed in inhaled air to prevent the user's mouth, throat and lungs from becoming unnecessarily dry and to aid in the respiratory process in general. Lack of moisture in inspired air may cause the user of the closed respiratory system to become dehydrated and uncomfortable. Dehydration and discomfort can lead to a more limited use of the respiratory system than desired and may cause the user to be distracted from the task or environment to which attention should be focused. Excessive dryness of the mouth and upper airways can also cause the user to inhale and exhale in an undesirable manner in an attempt to compensate for the dryness. Thus, it can be appreciated that it would be desirable to have a means for enhancing the moisture content of inspired air in a closed respiratory system. Because exhaled air contains moisture, it can be appreciated that it would also be desirable to utilize moisture in expired air in a closed respiratory system to enhance the moisture content of inspired air in the system.